Process for the thermal conversion of biomass to liquids

ABSTRACT

PCT No. PCT/CA93/00504 Sec. 371 Date Jul. 31, 1995 Sec. 102(e) Date Jul. 31, 1995 PCT Filed Nov. 24, 1993 PCT Pub. No. WO94/12592 PCT Pub. Date Jun. 9, 1994A high conversion of biomass, such as wood, sawdust, bark, or agricultural wastes, to liquids is obtained bypyrolysis at short reaction tines in a reactor capable of high heat transfer rates; the reactor being of the fluidized bed, circulating fluidized bed or transport type in which the conveying gas contains low and carefully controlled amounts of oxygen, allowing a reaction system with low concentrations of carbon monoxide or flammable gases with a resulting improvement in operating safety and potential improvement in thermal efficiency and capital costs. The oxidation steps may be carried out in one or two stages. The resulting liquid product may be used as an alternative liquid fuel or as a source of high-value chemicals.

FIELD OF INVENTION

This invention discloses an improvement on present methods of thermalconversion of biomass to liquids which eliminates safety problemspresent in the current practice due to high concentrations of carbonmonoxide and the handling of large volumes of explosive gases at hightemperatures. At the same time, potential savings in energy efficiencyare made possible.

BACKGROUND TO THE INVENTION

Current practice in obtaining high liquid yields from the thermaldecomposition of biomass at short reaction times (sometimes called"fast", "flash" or "rapid" pyrolysis) makes use of reactor types capableof high heat transfer rates to small biomass particles, in order toachieve the rapid heat-up rates necessary. Three of the most commonlyused types are the fluidized bed, the circulating fluidized bed or thetransport reactor. In the first two of these, hot gases and solids,normally inert, are brought into intimate contact with the biomassparticles. In transport reactors, either hot gas alone or a mixture ofhot gas and solids may be used. All of these reactors have in common arequirement for a significant gas flow, usually from 1 to 10 times theweight of biomass being processed. If pyrolysis is carried out in theabsence of oxygen, then the non-condensable gases formed will havesignificant contents of carbon monoxide, hydrogen, methane, and otherlight hydrocarbons or organics, and are of medium calorific value andcan be readily burned in air.

In order to preserve the high calorific value of these non-condensablegases, and to prevent the loss of organic liquid yield due touncontrolled oxidation reactions if air is present, it is currentpractice to use these gases as a recycle stream to supply the necessaryfluidizing or conveying gas for reactor operation. It is also currentpractice to heat this recycle stream indirectly in order to supply partor all of the heat necessary for the pyrolysis reaction.

The operation of a fluidized bed process has been described by Scott andPiskorz (1) (2). When poplar wood was used as feed in a fluidized bed ofsand at a temperature of 500° C. and a gas apparent residence time of0.48 seconds at a gas to-feed weight ratio of 3:1, the recycle gascomposition, on a moisture-free basis, was:

    ______________________________________                                        Hydrogen       1.49%        by volume                                         Carbon monoxide                                                                              47.83%                                                         Carbon dioxide 39.40%                                                         Methane        6.97%                                                          Ethylene, ethane, etc.                                                                       4.31%                                                          ______________________________________                                    

The yield of gas was 11.1%, of organic liquid 66.3%, and of char 11.8%,expressed as weight % of the moisture-free feed. The balance of 10.8%was water formed in the pyrolysis reactions.

It is apparent that a gas with such a high concentration of carbonmonoxide would be extremely toxic, and even small leakages or emissionswould pose a severe hazard to life. In addition, the gas can readilyform explosive mixtures with air due not only to the carbon monoxidecontent but also due to the content of other inflammable hydrocarbonsand hydrogen. However, in the prior art as described in thesepublications and also in our earlier Canadian Patent No. 1,241,541(September, 1988), it is specified that the gas used must beoxygen-free.

Pyrolysis with a transport reactor in which both gas and hot solids weremixed and transported with the biomass is described by Graham et al (3).Poplar wood was pyrolysed at 650° C. and 0.524 seconds apparentresidence time. Although gas recycle was not used in the reportedexperiments (nitrogen was used), if gas recycle had been practised aswould be expected in a larger scale unit, then, on an inerts-free basis,this reactor would give a gas for the above conditions having thefollowing analysis:

    ______________________________________                                        Hydrogen       4.18%        by volume                                         Carbon monoxide                                                                              63.10%                                                         Carbon dioxide 11.80%                                                         Methane        12.55%                                                         Ethylene, ethane etc.                                                                        8.37%                                                          ______________________________________                                    

Clearly, a gas of this composition would be hazardous in practice, beingboth highly toxic and readily forming explosive mixtures with air. Inthis work, the carrier gas used was inert. In subsequent disclosures byUnderwood and Graham (U.S. Pat. No. 4,876,108, Oct. 24, 1989 and U.S.Pat. No. 4,994,297, Feb. 19, 1991), three reactor systems were describedfor the preparation of fast pyrolysis liquids from wood or cellulose(4). In every example, and in all claims , it is specified that theprocess is to be carried out in the absence of oxygen. For the RTP unitdescribed, it is specified that inert gas is to be used together withsuspended particulate solids.

It is highly desirable to make use of the combustible non-condensablegases produced during pyrolysis in order to recover their heating valueand thereby improve the thermal efficiency of the process. If this isdone by use of a recycle stream of product gas after venting thepyrolysis product gas for use as a fuel supplement, then significantlylarge amounts of such gases must be cooled and heated to fulfill theirfunction as carrier or fluidizing gases for the reactor used. Given thehighly toxic and explosive nature of this recycle gas stream, it will benecessary in practice to build into the process extensive alarm systems,and emergency ventilation and fire prevention equipment at considerablecapital cost. A high degree of automation may be found to be required.Environmental considerations may add additional capital and operatingcosts. It is particularly difficult to prevent escape of some processgas at the point where biomass particles, must be fed to the system.Even if lock hopper feeders or other positive devices are used, anextensive purging capability must be installed.

SUMMARY OF INVENTION

We have discovered that pyrolysis of biomass in the type of reactorsdescribed can be carried out with little loss of yield of liquid or ofthe desirable high-value chemicals produced, by use of a carefullycontrolled oxidizing atmosphere. The recycle gas produced is low incarbon monoxide and is not combustible or flammable in air. All thenon-condensable combustible components are utilized efficiently toprovide a part of the required process heat. In previous processes inwhich partial combustion was used, as was the traditional practise inclassical destructive distillation of wood, or even in current practisesfor the preparation of charcoal in fluidized or moving bed reactors,yields of organic liquids are low, typically less than 35% as comparedto yields of 50% to 70% in non-oxidizing processes. For example, thecompany literature describing the fluidized bed partial oxidationprocess of BIO-Alternative SA for the preparation of solid, liquid andgaseous fuels by the "combustion of carefully dosed pyrolysis gas"reports typical yields from waste wood as 43 to 53% charcoal, 15 to 25%oil, and 18 to 25% gas (5).

In our preferred method as described below, oxidation is used to destroycarbon monoxide and other toxic or flammable gases while still achievingliquid yields comparable to non-oxidizing pyrolysis processes, andrecovering the heat content of these gases. As a result, a pyrolysis gaswhich is of low toxicity and which is not capable of forming explosivemixtures is produced for use as a conveying or fluidizing gas.

More significantly in our preferred embodiment of the pyrolysis process,the yields of high value chemicals are affected to only a minor degree.In particular, the yield of hydroxyacetaldehyde, one of the mostabundant of these compounds, is reduced by only a small amount, lessthan 10% in a properly operated process. It was not known in theprevious art that this was possible, while still carrying out apyrolysis in an oxidizing atmosphere.

Accordingly, in this invention there is provided a process for theconversion by thermal means at short reaction times of lignocellulosicmaterials (biomass), or products derived from biomass which containcellulose, hemicellulose and lignin fractions as major components inwidely variable amounts, to produce a high yield of organic liquidproducts; the process being carried out under conditions which oxidizeselectively combustible non-condensable gases generated in the pyrolysiswhile oxidizing only a minor portion of other organic products, with theresult that the large amount of recycle gas used in the process forconveying or fluidization is of low toxicity compared to that used innon-oxidizing processes, and is not capable of forming explosivemixtures with air.

The process defining our invention comprises the following steps.

(a) adding sufficient air to the stream of recycle process gas to beused for conveying or fluidization to provide an excess of oxygen of 0%to about 200% of that required for combustion of combustible gascomponents, such as carbon monoxide, hydrogen, methane, ethane,ethylene, and other light hydrocarbons or volatile organic compounds;

(b) passing the resulting air-recycle gas mixture through a conventionalcatalytic converter to oxidize the carbon monoxide and other gases inthe recycle process gas. Some preheating of this gas could be done ifnecessary to make the oxidation step more efficient;

(c) reheating the oxidized recycle gas together with any excess oxygento a sufficiently high temperature to supply the required heat to apyrolysis reaction;

(d) subjecting a biomass feed after grinding to small particles (e.g.sawdust) and drying to less than to about 10% moisture content to shortresidence time pyrolysis within the conditions of temperature about 400°C. to about 650° C., gas and volatiles residence time of about 2 secondsor less and substantially atmospheric pressure to form a product gas;

(e) condensing from the product gases an organic liquid product togetherwith a majority of the water after removal of char or unreacted solids;and

(f) recycling the non-condensable gases and inerts to step (a).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic flow sheet of one embodiment of the process of theinvention. A detailed mass balance is provided for gas flows utilizingthe process of this invention, on the basis of a unit of 100 dry kg ofpoplar sawdust feed, with a recycle gas/sawdust weight ratio of 4:1, toa pyrolyzer operating at 490° C. Gas yield and composition are based ona volatiles residence time of 0.48 seconds in the pyrolyzer. Sufficientair is added to the recycle gas stream as shown to represent 130% of thestoichiometric requirement to oxidize completely the carbon monoxide,methane, and other non-condensable hydrocarbon gases and volatileorganic yapours. Complete oxidation is assumed to occur in the oxidizer.The unreacted oxygen added in the air in excess of this stoichiometricrequirement (30%) is assumed to react with the sawdust in the pyrolyzerto form water and carbon dioxide or carbon monoxide (in a 4:1 ratio).Temperatures given are obtained from energy balances on the pyrolysissystem. A catalytic oxidation unit has been assumed, using a platinumcatalyst, and operating at 575°-750° C.

FIG. 2 is a graphical representation of the performance of the processof the present invention. It is apparent from this graph that the carbonmonoxide content of the recycle gas can be reduced to a low level by theuse of carefully controlled amounts of oxygen, conveniently added asair, coupled with an efficient oxidation unit capable of oxidizing lowconcentrations of the combustible gases. Yields of the importantproducts, such as organic liquid, or of valuable components of theliquid, such as hydroxyacetaldehyde, are not significantly affectedunless more oxygen than necessary is used by a factor of about two.However, it is also apparent that some valuable but reactive components,such as the hydroxyacetaldehyde, suffer a very rapid loss in yield ifoxygen above the amounts preferred in the practice of this invention areused. Conversely, it is also apparent from FIG. 2 that smalldeficiencies in the stoichiometric requirement of oxygen could betolerated if some level of carbon monoxide concentration in the recyclegas is acceptable above the minimum obtainable by the practice of thisinvention in its preferred form. In this latter case, the pyrolyzer gasstream shown in FIG. 1 contains essentially zero oxygen content, andthere would be no oxidation reaction occurring in the pyrolyzer. Processtemperatures as shown in FIG. 1 would need to be adjusted to compensatefor the reduction in energy released due to the lower extent ofoxidation reactions. Other consequences of operation with deficienciesor excesses in the supply of stoichiometric oxygen will be apparent fromFIG. 2.

GENERAL DESCRIPTION OF INVENTION

A pyrolysis reactor is operated generally at the conditions we havedescribed in our earlier publications and patent, that is, at conditionsoptimal for maximum yield of organic liquid, a temperature from about400° C. to about 650° C., an apparent vapor residence time of less thanabout 2 seconds and at substantially atmospheric pressure. An amount ofair is added to the recycle gas stream which is from 0% to about 200% inexcess of that required to stoichiometrically completely oxidize thenon-condensable components of the recycle gas stream. A preferred amountis from about 0% to about 100% excess air. The recycle gas in the steadystate is very dilute with respect to carbon monoxide or othercombustibles. A flow sheet showing the mass balance which is achieved inthe present invention is given in FIG. 1, based on a conceptual plantwith 100 kg per unit time of dry poplar sawdust feed, containing 6%moisture, and operation at optimal conditions of temperature andresidence time, but with 30% excess air added in excess of therequirements for combustion of components in the recycle stream. Therecycle gas available for use in the process for fluidizing or conveyingafter condensing out the organic liquids and a majority of the waterformed, for this example, has a composition as given below,

    ______________________________________                                        Hydrogen           0.04%     by volume                                        carbon monoxide    1.44%                                                      Carbon dioxide     29.87%                                                     Nitrogen           68.25%                                                     Methane, other hydrocarbons                                                                      0.40%                                                      ______________________________________                                    

It is apparent that this gas has a low level of toxicity, and is notcapable of forming explosive mixtures with air. Further, as shown inFIG. 1, a side stream containing no carbon monoxide can be taken aftercooling for pneumatic conveying of the feed or for purging of the feedsystem, to give a completely non-toxic feed system.

The air is introduced into this recycle stream preferably in an amountconstituting a small excess over the stoichiometric requirement. This isnecessary if it is desired to ensure a minimum content of carbonmonoxide in the recycle gas. If a deficiency of oxygen were used, then abuild-up of carbon monoxide concentration would occur to a new and muchhigher steady-state value. These relationships are shown in FIG. 2, inwhich both predicted and experimental values are shown, from tests withboth hardwoods and softwoods. The curves shown will be at a littlehigher values for lower gas/wood ratios, and at lower values for highergas/wood ratios.

The recycle gas composition given above is comparable to the exhaustemission from a gasoline internal combustion engine. The preferredoxidation method of this invention, therefore, is by the use of acatalytic oxidation unit entirely comparable in function to those usedon automobile exhausts. This technology is well developed, flexible andefficient. For example, platinum -based catalysts for oxidation ofexhaust emissions are designed to function to give 98% or betteroxidation of carbon monoxide and hydrocarbons at carbon monoxideconcentrations of 1% to 3% and at hydrocarbon concentrations up to 2%.Temperatures above 500° C. are adequate for carbon monoxide oxidation,but higher temperatures are required for complete oxidation ofhydrocarbon gases. These values are within the ranges shown for therecycle gas in FIG. 1. Therefore, the same technology as that applied toengine exhaust emissions can be equally well adapted to the purposes ofthis invention. If the emission gases contain little or no sulfur, as isthe normal case from biomass -derived materials, then catalystcompositions other than platinum are also possibilities.

It is also possible:to use a non-catalytic oxidation unit, either as aseparate high temperature unit, or as a part of the preheating process.However, because of the low carbon monoxide, hydrocarbon and oxygenconcentrations in the recycle gas, either the temperature must be high,for example, above 900° C., or the reaction time must be long, toachieve a high degree of oxidation. Therefore, because of the provenperformance of catalytic oxidation units, and their lower operatingtemperatures, this method is the one preferred for this invention.

The catalytic oxidation step can best be carried out in a separate unit.The heat available from this oxidation increases the temperature of therecycle gas stream of the order of about 150° C. to about 250° C. Theadditional heat needed for the pyrolysis reaction is added indirectlythrough a conventional fired preheater. While it is theoreticallypossible to fire additional fuel and air directly into the recycle gasstream before or after the oxidation unit, this practice is generallynot desirable because of the additional water introduced into the systemwhich will be condensed out with the organic bio-oil, and degrade itsheating value if it is to be used as a fuel, or make the recovery ofchemicals more difficult. It is therefore the preferred embodiment ofthis invention to add the required additional process heat indirectly.

If any excess oxygen is introduced at the oxidation unit, it enters thereactor with the recycle gas and reacts there with the biomass or withthe volatile products. Our tests with this dilute oxidizing system haveshown that the carbon oxidized in the reactor by the excess oxygenappears approximately as 20% carbon monoxide and 80% carbon dioxide, sothat it contributes only in a minor way to the carbon monoxide contentof the recycle gas. This is also a unique feature of the presentinvention. The heat generated by this oxidation, which generallyconsumes less than about 5% of the biomass fed, contributes to reducingthe indirect heat that must be added to the conveying or fluidizing gasstream, and represents an efficient heat source. The process asdescribed above is readily controlled by careful adjustment of theoxygen content of the gas entering the reactor. It is also of interestthat in some tests with a non-oxidizing atmosphere in which a gas with ahigh content of carbon monoxide was introduced into the pyrolysisreactor, gaseous iron and nickel carbonyls were detected. These highlytoxic compounds could appear in the product liquid. However, in thepresent invention, the concentration of carbon monoxide is too low inthe recycle gas to form detectable amounts of these carbonyls.

The preferred level of oxygen entering the pyrolysis reactor in thisinvention is from 0% to about 2.0%, with a favorable range of about 0.2%to about 0.5%, by volume. At this level of oxygen, only minor changes inthe compositions and yields given for the 0.0% oxygen case are to beexpected.

EXAMPLES Example 1

Western hemlock sawdust was pyrolyzed at three levels of oxygen contentin the fluidizing gas entering a fluidized bed reactor containing sandas an inert heat carrier solid. The results obtained are shown in Table1 below.

                  TABLE 1                                                         ______________________________________                                        Pyrolysis of Western Hemlock Sawdust                                          Particle size -0.5 mm, Moisture 5%                                                            nitrogen/ nitrogen/                                           Atmosphere      air       air      nitrogen                                   ______________________________________                                        Temperature °C.                                                                        455       445      460                                        % Oxygen in gas 12        1.6      0.0                                        Oxygen/wood ratio (wt)                                                                        1.8       0.3      0.0                                        Gas, % of dry feed                                                                            67.98     17.87    9.77                                       Water, % of dry feed                                                                          27.48     17.35    11.78                                      Organic liquid  32.32     45.13    47.46                                      Char            12.65     23.92    24.36                                      Total recovery  140.43    104.27   93.37                                      Carbon monoxide, wt % feed                                                                    14.15     4.73     3.18                                       Carbon dioxide  52.77     12.94    6.01                                       Methane         0.27      0.16     0.17                                       Other hydrocarbons, etc                                                                       0.79      0.04     0.41                                       Hydroxyacetaldehyde                                                                           2.10      4.35     8.01                                       (wt % of dry feed)                                                            Formic acid     3.13      2.79     2.62                                       Acetic acid     0.68      0.88     0.99                                       ______________________________________                                    

It is apparent from a comparison of the results of the three experimentspresented in Table 1 that the yield of organic liquid decreased and thewater produced increased as the oxygen content of the gas entering thepyrolyzer increased. At an oxygen content of 1.6% by volume in the gas(representing the excess oxygen supplied over the stoichiometricrequirement for complete oxidation of the combustible components of therecycle gas), the overall liquid yield was not decreased although theorganic content is a little lower and the water content considerablyhigher at these conditions. However, the liquid obtained may stillprobably serve as a low quality alternative fuel oil. The yield of avaluable component of the liquid, hydroxyacetaldehyde, was reduced,however, to only about one half of that obtained when there is no oxygenentering the fluid bed pyrolyser, that is, when the stoichiometricamount (or a little less than a stoichiometric amount) of oxygen isadded to the recycle gas before the oxidation step. At a high oxygenlevel of 12% in the gas entering the pyrolyzer, organic liquid yield wasconsiderably reduced and the water content of the liquid product was toohigh for use as a fuel oil. Also, the yield of hydroxyacetaldehyde wasreduced to only 26% of that obtained with zero or low oxygen content.Additional analysis of the liquid components (not shown) indicates thatthe composition of the organic liquid fraction also changessignificantly with respect to other components of the liquid as thedegree of oxidation increases and this may result in a degraded liquidproduct for many potential uses.

Example 2

A series of tests was carried out on a hardwood sawdust (poplar) at 489°C. to 504° C. and with a small excess oxygen additions over thatrequired for stoichiometric oxidation of the non-condensable combustiblecomponents of the recycle gas stream. The results of these tests areshown in Table 2 below, and cover a range of 0% to about 200% excessoxygen (that is, 100% to 300% of the stoichiometric oxygen requirement).Tests were carried out in a continuous bench scale fluidized sand bedoperating at essentially atmospheric pressure. It is apparent from theresults contained in this Table 2 that even with the higher excessoxygen amounts, the reduction of organic liquid yield was small, about5%. However, the water content increased somewhat, and the yield of akey valuable organic component, hydroxyacetaldehyde, was significantlyreduced at the higher oxygen levels, but not at the lower ones.

                  TABLE 2                                                         ______________________________________                                        Pyrolysis of Poplar Sawdust                                                   Particle size -0.5 mm, Moisture 5, 5%, 0, 5 sec residence-time                Atmosphere  nitrogen nitrogen/air                                             ______________________________________                                        Temperature, °C.                                                                   504      490     491   489   493                                  % oxygen in inlet                                                                         0.00     0.25    0.35  0.56  0.83                                 gas                                                                           Oxygen/wood, wt.                                                                          0.00     0.033   0.042 0.075 0.089                                ratio                                                                         Gas, % of dry                                                                             10.4     16.4    16.0  20.8  21.8                                 feed wt.                                                                      Water (product)                                                                           14.1     8.7     11.8  9.9   15.2                                 Organic liquid                                                                            60.7     59.0    58.6  57.2  57.5                                 Char        11.1     10.7    10.4  10.2  9.9                                  Total Recovery                                                                            96.3     94.8    96.8  98.1  104.5                                Carbon monoxide,                                                                          3.34     4.75    4.15  5.13  4.13                                 % feed                                                                        Carbon dioxide                                                                            5.51     9.92    10.38 13.87 15.49                                Methane     0.25     0.29    0.29  0.31  0.31                                 Other hydrocarbons                                                                        1.30     1.46    1.15  1.53  1.85                                 Hydroxyacetal-                                                                            7.93     8.04    5.43  5.00  4.48                                 dehyde                                                                        (wt. % dry feed)                                                              Formic acid 2.38     2.19    2.00  2.38  1.95                                 Acetic acid 2.09     1.86    1.67  1.92  1.78                                 ______________________________________                                    

The results shown in Table 2, together with those given in Table 1, showthat a small excess of oxygen over that required for combustion ofcarbon monoxide, methane, and other gases in the recycle stream, is notdetrimental to yields of the desired products. However, the addition ofthe oxygen must be carefully controlled to prevent loss of yield anddegradation of the liquid product by excessive oxidation. A small excessof oxygen over that required stoichiometrically for gas combustion wouldhelp to ensure high efficiency in the oxidation unit. However, as shownin FIG. 2 and in Tables 1 and 2, the ideal case is to use just the exactamount of oxygen required, that is, the stoichiometric amount, so thatthe oxygen content of the gas from the oxidation unit to the pyrolyzerequals or approaches 0%.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention provides a novelprocedure for the thermal conversion of biomass to liquid products inwhich oxidation of combustible gases in a recycle stream is effected.Modifications are possible within the scope of this invention.

REFERENCES

1. J. Piskorz, D. Radlein and D. S. Scott, "On the Mechanism of theRapid Pyrolysis of Cellulose" J.Anal. Applied Pyrolysis, 9, 121-137,(1986).

2. D. S. Scott, Canadian Patent No. 1,241,541 "Pyrolysis Process"September, 1988.

3. R. G. Graham, B. A. Freel and M. A. Bergougnou, "The Production ofPyrolyric Liquids, Gas and Char from Wood and Cellulose by FastPyrolysis" in "Research in Thermochemical Biomass Conversion" A. V.Bridgwater and J. L. Kuester (Eds.), Elsevier Applied Science Publ.(London) (1988) pp 629-641.

4. Gary Underwood and Robert G. Graham, U.S. Pat. No. 4,876,108 "Methodof Using Fast Pyrolysis Liquids as Liquid Smoke", Oct. 24, 1989, alsoU.S. Pat. No. 4,994,297, Feb. 19, 1991.

5. BIO-ALTERNATIVE SA--Recycling forest and agricultural wasteproducts--a Swiss technology for recuperating biomass energyNeuchatel/Switzerland, March 1989/1.

What we claim is:
 1. A process for the thermal conversion of biomass orbiomass derived substances to high yields of liquid products employing ashort-residence time reaction, the process comprising the steps of:a)adding to a stream of recycle process gas to be used for conveying orfluidization in a short-residence time pyrolysis reactor sufficientoxygen-containing gas to give an excess of oxygen of 0% to about 200% ofthat required for combustion of combustible gas components to form anoxygen-containing gas-recycle gas mixture, b) passing theoxygen-containing gas-recycle gas mixture through a catalytic converter,with preheating if necessary, to oxidize combustible gas components inthe recycle gas mixture, c) preheating the oxidized recycle gas togetherwith any excess oxygen to a sufficiently high temperature to supply therequired heat to a pyrolysis reaction, d) subjecting a biomass feed,after grinding and drying the particles to less than about 10% moisture,to short-residence time pyrolysis in the presence of 0 to about 2% byvolume of oxygen at temperatures of from about 400° C. to about 650° C.,gas plus volatiles residence times less than about 2 seconds, and atpressures substantially atmospheric to form a product gas, e) condensingfrom the product gases an organic liquid product together with waterafter removal of char or unreacted or inert solids to provide saidrecycle process gas, and, f) recycling said recycle process gas to step(a).
 2. The process according to claim 1, wherein said amount of oxygencontaining gas added to said recycle process gas is from about 10 toabout 100% excess.
 3. The process according to claim 1, wherein saidamount of oxygen containing gas added to said recycle process gasresults in an oxygen concentration in said pyrolysis reaction of about0.2 to about 0.5% by volume.
 4. The process according to claim 1,wherein the short residence-time pyrolysis is effected in a fluidizedbed of inert solids.
 5. The process according to claim 1, wherein theshort residence-time pyrolysis is effected in a circulating fluidizedbed of inert solids.
 6. The process according to claim 1, wherein theshort residence-time pyrolysis is effected in a transport reactor inwhich biomass is conveyed together with gas or gas/solid mixtures in anyof an upflow, downflow or cyclonic mode.
 7. A process according to claim1, wherein the amount of added oxygen is sufficient to oxidizesubstantially the majority of the non-condensible combustible gases inthe recycle gas stream before it enters the pyrolysis reactor, but whichresults in essentially a zero oxygen content in the gas entering thepyrolyzer, so that pyrolysis occurs under non-oxidizing conditions. 8.The process according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the liquidcondensate product is used as an alternative fuel oil.
 9. The processaccording to claim 1, 2, 3, 4, 5, 6 or 7, wherein the liquid condensateproduct is used as a source of hydroxyacetaldehyde.